Coaxial directional coupler



Oct. 27, 1953 R. E. HENNING COAXIAL DIRECTIONAL COUPLER 2 Sheets-Sheet 1 Filed Jan. 27 1950 k 'IIIIIIIIIIIIIIIIIIIIIII/ln INVENTOR fiz/oozp if/f/v/w/va M TBRNEY Oct. 27, 1953 R. E. HENNING COAXIAL DIRECTIONAL COUPLER 2 Sheets-Sheet 2 Filed Jan. 27, 1950 NJ INVENTOR S ATTORNEY Patented Oct. 27, 1953 UNITED STATES PATENT OFFICE Rudolf Henning,1New York, N. assigfnor to The Sperry"CorporatiomGreat Neck, N. Y., a

corporation of Delaware iipplicationilanuaryzl, 1950,'Serial'No. 140,833

4 Claims. 1

This invention relates to electromagnetic energy couplers between ultra high frequency energy transmission lines, and in particular relates to directional couplers of the coaxial line type.

-A directional coupler is 'a device which, when inserted in tandem in -"a transmission line -on which there exists waves travelling in both directions, delivers to a pair of terminals located in an auxiliary transmission line a voltage which islarge'ly-a function-of the amplitudeof the wave travelling in one preferred directionin the main transmission line, and relatively independent of the wave going in the opposite direction.

That part of the directional coupler which is inserted in tandem with the transmission line is called the'primary or mainline. It possesses a set of main line input terminals at which the wave travelling in the preferred direction enters. The other set of terminals is called the mainline output terminals.

The secondary or auxiliary line has, in addition to its output terminals, a termination element. This element serves to absorb theenerg-y coupled into the auxiliary line which is travelling in the wrong direction and further serves to provide a good impedance match when looking into the auxiliary line from its output terminals. The main and auxiliary lines are connected together by a coupling mechanism which gives the coupler its directive properties.

'The operation of adirectional coupler may be described in-terms of its coupling, directivity, and band width. The coupling specifies that fraction of the power proceeding in the preferred direction in the main line which is delivered to the auxiliary line output terminals. More precisely, the coupling, which may be expressed either as'a voltage-or power ratio, is defined as the-ratio of voltage (or power) delivered to a matched detector'at the auxiliaryline output terminals to the voltage (or power) deliveredto the couplers" main line input terminals, providedthe mainline output terminals are terminated in a matched load.

The directivity of a directional couplerfis the ratio of voltage iorpower delivered to the termination element in thezauxiliar-y line'to the voltage (or power) delivered to the matched detector in the auxiliary line when power is fiow ing in the main line .in the preferred direction with a matched load connected to the :main line output terminals. Con-ventionally, #this ratio is expressed in decibel notation. i v

Theloand width of 'a directional coupler may be defined as the frequency operatingrange over which a directional coupler maintains a preassigned minimum directivity and coupling. In an ideal directional coupler the directivityand' coupling are independent of the operation frequency.

One type-of -directional coupler well known "in the art is the Bethe hole-coupler. Such-a ecup'ler consists of two sections of transmission line, such'a's coaxial line, having a common outer conductor wall section. The coupling mechanism consists of a small aperture "in 'thiscommon'wall section. The coupling mechanism 'of an ideal Bethe hole coupler may he '-explained as follows:

small, round hole, connecting two parallel coaxial transmission lines will, when excited *by constant field, such as that of the TEM mode of coaxial propagation, excite energy flow in-the secondary or the auxiliary line. 'In particular, for energy travelling in the forward direction in the primary or main line, =such'a hole will cause some energy "to travel equally in' both directions in the secondary line, due to excitation of the hole by the electrical component of the TEM mode. Simultaneously, four times the above energy ("twice the amplitude) will flow only in the reverse direction in the secondary line due to excitation of the magnetic component of the TEM mode. In order to achieve good .dir'ectivity (a minimum of -'energy travelling in the forward the forward direction and amplitude addition in I the backward direction.

In many applications it is undesirable to have the main and auxiliary line section's oriented at an angl'e to eachpther. Instead, it-is usually preferred to have'them positioned parallel to each other. Bethe has shown that, theoretically, such a parallel line directional coupler is possible to design if =thecoupling aperture is a thin slot, infinite in length instead of the finite circular-aperture described above. '-In such a thin slot type coupling aperture the magnetic and electriccouplings are equal in-amplitude and the-desired directivity is achieved without the necessity of "adjusting the angle between the main and auxiliarylines. However, by *making'the slot very thin the coupling is extremely low. Furthermore, in practice, a thin slot which is infinite in length is impossible to achieve.

It is therefore an object of the present invention to provide a coaxial type directional coupler whose line sections are oriented parallel to each other.

It is another object of the present invention to provide an axial-slot type directional coupler having a relatively large coupling while maintaining frequency insensitive directivity.

It is a further object of the present invention to provide an axial-slot type directional coupler having a relatively large coupling while introducing a minimum amount of reflection in the main and auxiliary lines.

Yet, another object of the present invention is to provide a closely coupled, axial-slot type directional coupler in which the ratio of the induced electric and magnetic fields is substantially constant and unity regardless of the frequency within the operating range.

Another object of the present invention is to provide means for compensating the limiting effects of a finite length coupling slot in an axialslot type directional coupler.

Still another object of the present invention is to provide means for increasing the electric coupling between the main and auxiliary line sections of a coaxial line type directional coupler.

A further object of the present invention is to provide an axial-slot type directional coupler having a relatively large coupling while maintaining the characteristic impedance of the main line constant.

In accordance with the present invention, a directional coupler of the coaxial line type is provided having its main and auxiliary line sections oriented parallel to each other with the inner or outer conductor of the main line having a common wall section with the inner or outer conductor of the auxiliary line. Coupling is provided by means of a long, thin slot in the common wall section. A raised, conductive section is provided along the inner conductor in order to compensate for the limiting efiects due to the finite length of the slot. In addition, the raised section aids in maintaining the original characteristic impedance of the main coaxial transmission line constant, thereby minimizing the standing wave ratio therein.

These and other features of the present invention will become apparent from a consideration of the following specification and attached drawings, wherein:

Fig. 1 shows a sectional view of a coaxial line type directional coupler utilizing one embodiment of the present invention;

Fig. 2 is a cross-sectional view taken along the line. 2-2 of Fig. 1;

Figs. 3 and 4 are schematic representations of Fig. 8 is a cross-sectional view taken along line 8-8 of Fig. 7.

Referring now particularly to Fig. l,.there is shown coaxial line section it having inner conductor H and outer conductor l2 oriented parallel with coaxial line section Ha having inner conductor l5 and outer conductor iii. Outer conductors l2 and 16 are contiguous, forming a common wall section between coaxial line section it and it. Coupling between line sections Id and H3 is achieved by means of an axially directed slot 25 positioned in the common wall section formed by outer conductors i2 and it. This slot 2.? is directed along the axis of coaxial lines it and i i and has a finite length-to-width ratio. A bar providing a conductive ridge 2i is mounted on inner conductor i 1 along the length of slot This conductive ridge 2i extends in a direction toward slot 2e as is seen more fully in Fig. 2 which is a section view taken along line 22 of Fig. 1. Tapered sections 22 and 23 are provided at each end of conductive ridge 2| in order to minimize any reflections created in main transmission line It thereby.

In operation, axially directed slot couples energy from main line section iii to auxiliary line section it. By virtue of its finite length-towidth ratio the coupling therethrough is pr.- dominantly magnetic. In order for the coupler to achieve a high degree of directivity, is essential that the electric and magnetic couplings be made equal to each other. By decreasing the Width of slot the amount of magnetic coupling is reduced more rapidly than the amount of electric coupling.v In fact, as mentioned above, Bethe has shown that if the length-towidth ratio were infinite the electric and magnetic couplings would be equal. Reducing the width of the coupling slot, however, reduces the overall coupling between lines i2 and is. In many cases it is desirable to provide the maximum. amount of coupling possible while still providing a high degree of directivity. To achieve this result, the conductive ridge 21 is added in accordance with the present invention. This conductive ridge reduces the effect of distance between the inner conductor ll of coaxial line section it and the inner conductor 15 of coaxial line section is. This results in an increase of electric field strength in the region of axial slot 2%) and thereby increases the electric field coupling between line sections Ill and Hi. If conductive ridge 2i is properly dimensioned this increase in electric field coupling results in an equal amount of electric and magnetic coupling between line sections it and 1% thereby resulting in high directivity. In practice the width of the ridge 2!, i. e., the dimension measured transversely of the plane defined by the parallel longitudinal axes of the coaxial line sections, is made less than the diameters of the inner conductors l i and E5 in contrast to the width of the coupling slot which is greater than the diameters of the inner conductors. It should be noted that this desirable result is achieved without reducing the overall coupling between the two coaxial line sections.

As might be expected, any opening in the outer conductor 52 of transmission line section iii, such as is provided by slot 29, tends to raise the characteristic impedance of the transmission line in the region of the coupling aperture. This, of course, is due to the increase in the equivalent diameter of the outer conductor, in this region. Naturally, any change in the characteristic impedance produces a discontinuity in the transmission line with the result that energy reflections are set up at the point of the disabuser continuity. Such energy reflections create :a large standing wave ratio with its resulting low efilciency and decrease in power handling 'capabilities.

By introducing the conductive ridge 2] in the region of axial slot '20 the .eiiective diameter of the inner conductor is also increased. This, of course, tends to make the characteristic impedance of the transmissionline constant. Fig-s. 3 and 4 aid in showing that, not only :does the conductive ridge aid in equalizing the electric and magnetic coupling, but at the same timegincreases the eiiective diameterof the inner conductor sufficiently to make the characteristic impedance of the main transmission line constant.

Fig. 3 is a schematic representation of the lumped constant equivalent circuit for 1a coupler employing both electric and magnetic coupling. The equivalent circuit is shown on a normalized basis. Alternating current generator ilil having internal impedance 3| is connected to the input terminals 32 and 33 of main transmission line34. This transmission line is terminated with impedance 35. Auxiliary line 40 is coupled vboth electrically and magnetically to main transmission line 3 5. The series inductance of the coupling aperture is represented by inductor 4i and the shunt capacity is represented by capacitor d2. Auxiliary transmission line 40 is terminated by impedances 43 and 44.

The circuit shown in Fig. 3 can be transformed into a bridge circuit as shown in Fig. 4. It is evident from this representation that if the inductance of inductor Al is equal to the capacity of capacitor 42 (on a normalized basis) .no current will flow in resistor 44, which represents the termination of the auxiliary line section 80. It should be remembered that this is the condition for infinite directivity since by definition directivity is the ratio expressed in decibels of voltage delivered to a termination element (resistor 44) in the auxiliary line 49 to the voltage delivered to the matched detector (resistor 43) in the auxiliary line 40 when power is flowing in the main line 34 with a matched load (resistor 35) connected to the main line output terminals. Thus, if no current flows in resistor 44, by definition, the directivity is infinite. It should be noted that this condition is independent of frequency.

Coincident with this directive property of'this four-terminal network is the fact that the balanced bridge condition presents a matched impedance to the source as measured across terminals 32, 33. Thus, the lumped constant analysis leads to the conclusion that the highest intrinsic directivity and a good match of the coupling elementto the primary line will exist coincident with each other. It is thus apparent that the excessive magnetic coupling caused by the finite length-to-width ratio of the coupling slot is equivalent to the fact that the inductance of inductor All is greater than the capacity of capacitor 12. By adding the proper amount of capacity to the system at the point of coupling the bridge is balanced.- This additional capacity, in accordance with the present invention, results from the insertion of the raised section or conductive ridge 2| along the inner conductor I of mainline I0. w

, While the embodiment of the present invention illustrated in Figs.) :and 2, show the con ductive ridge employedin the main line only, Fig. 5 illustrates another embodiment of the "present invention in which the conductive ridge is -em-.

ployed both the main and auxiliary line sections. Reference to Fig. 5 shows main transmission line 60 having inner-conductor -61 and outer conductor 62 oriented parallel to auxiliary transmission "line 64 having inner conductor 65 and outer conductor -56. Outer conductors S2 and G6 are contiguous, forming acommon wall section between coaxial line sections Bil and 15 1. Coupling between the line sections '60 and M is achieved by means of an axially direc'ted's'lot 10 positioned in the common wall section formed b'y outer conductors 62 and B6. Main transmission line fio is terminated at one end in a conventional male coupling N10 'to which a matched resistor 1-05 is shown connected schematically. The op-- po'site end is terminated in a conventional 'female coupling F0! to which generator -I OS with' internal impedance T01 is shown connected schematical-ly.

As in the previous embodiment, this slot T0 is directed along the axis of main line and auxiliary line 64 and has a finite length-to-width ratio. Gne end of auxiliary line section 64 is terminated -in matched impedance element H. This terminating element serves to absorb any incident energy thereon preventing reflections in line section 6E. The opposite end of auxiliary line section M is provided with a rightangle section 12 is terminated ina-conventional male -'output coupling 32 to which a matched load 13 is shown connected schematically.

Conductive ridge *8] is mounted on the inner conductor 6 along the length of slot Til. This conductive ridge 8! ex't'endsin a'direction toward slot Til *as is seen more fully in Fig. 6 which is a section view taken along line '6-5 "of 'Fig. '5. Tapered sections '82 and 83 are provided at each end of conductive ridge Bi in "order {to minimize any reflections'created due to an impedance dis continuity caused by the *presenceof element iii in main transmission line 69. Conductive ridge 9-1 "is mounted on inner "conductor b5 along the length of slotifl. This ridge d! also extends in a direction toward slot '50. Tapered sections 92 and 93 are provided at each end of this ridge element in order to minimize reflections in iliary line 64.

:As in the previous embodiment, axially -'di rected slot '10 couples energy from main line section to auxiliary line section 64. This coupling is predominantly magnetic due to the finite length to-'width ratio of the long, thin axial slot type coupling aperture. Conductive ridgesBl and 9-] serve to increase the electric field strength in the region of the coupling aperture, and thereby increase the amount of electric coupling. This enables the overall electric coupling to equal the magnetic coupling thereby achieving a high degree of directivity. This high directivity is achieved without reducing the amount of magnetic coupling "resulting from slot 70., thereby permitting the overall coupling to remain relatively large. As discussed above, the conductive ridges also tend to increase the eiieetive diameter of the inner conductors in the region of the coupling slot. This o'fisets the in-- crease ineffective diameter of the outer conductor due to the coupling slot, maintaining constant characteristic impedance in the main and auxiliary transmission .lines. This, of course, results in a smooth flow of energy with a .minimum amount. of reflection introduced by the coupling mechanism. .As shown above, this desirable result'is achieved by virtue of com- 7 pensating elements BI and 91, their compensating efiects being independent of operating frequency.

Fig. 7 illustrates another form of a coaxial line type directional coupler utilizing the present invention. In this embodiment coaxial line section H0 is formed of inner conductor H2 and conductor H3. Coaxial line section H4 utilizes conductor H3 as its inner conductor and outer conductor H5. Thus, conductor H3 forms a common wall between vested coaxial line sections H0 and H4. An axially directed coupling slot E20 is formed in the common wall I 13. Conductive ridge [2! is mounted on inner conductor I I2 along the length of slot I20, and has tapered sections I22 and I23 at each end thereof. Generator 139 having internal impedance I3! is shown connected schematically to one end of coaxial line section iii), and termination element I32 is shown connected to the opposite end thereof. Termination elements I33 and I34 are shown connected to opposite ends of coaxial line section The relative positioning of these elements is better seen in Fig.8 which is a cross-sectional view taken along line 8-8 of Fig. '7. This figure not only shows the nested relationship of conduotors H2, H3 and US, but shows conductive ridge I2! extending radially outward from inner conductor H2 toward coupling slot 529. 7

As in the previous embodiments a portion of the energy introduced into coaxial line section Hi3 by generator E39 is coupled into coaxial line section us by coupling aperture in. Because of the finite length-to-width ratio of the aperture l2 more magnetic coupling is achieved than electric coupling. Conductive ridge E21 overcomes this difficulty by introducing an additional amount of electric coupling, thereby permitting a high degree of directivity.

Although only a single conductive ridge is shown in the embodiment of Fig. '7, it is understood that an additional ridge mounted on the outer conductor H5 could also be utilized as in Fig. 5.

It is thus seen by the introduction of compensating elements in a coaxial line type directional coupler whose main and auxiliary lines are oriented parallel and whose coupling is provided by an axially directed slot, high directivity may be achieved over large frequency operating ranges. In addition, such compensating elements permit relatively large coupling between the main and auxiliary transmission lines. Not only do these compensating elements increase the amount or electric coupling thereby providing a high degree of directivity, but they serve to maintain the characteristic impedance of both the main andiauxiliary transmission lines cone stant, thereby minimizing any energy reflections introduced by the coupling aperture.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: e

1. An ultra-high-frequency directional coupler comprising a first section of coaxial transmission line having an inner and outer conductor, a sec ond section of coaxial transmission line havin an inner and outer conductor, said second section being oriented parallel to said first section and having a common outer conductor wall section therewith, said common wall section having an axially directed coupling slot located therein, and a first thin conductive strip of width substantially less than the diameters of said inner conductors secured along one edge to the inner conductor of said first section, said strip extending longitudinally along the inner conductor opposite the coupling slot, a second thin conductive strip of Width substantially less than the diameters of said inner conductors secured along on edge to the inner conductor of said second section, said second strip extending longitudinally along the inner conductor of said second section opposite the coupling slot, said strips projecting radially toward each other, the ends of said strips being tapered, the width of said coupling slot being greater than the diameters of said inner conductors.

2. An ultra-high-frequency directional coupler comprising a first section of coaxial transmission line having an inner and outer conductor, a second section of coaxial transmission line having an inner and outer conductor, said second section being oriented parallel to said first section and having a common outer conductor wall section therewith, said common wall section having an axially directed coupling slot located therein, and a first thin conductive strip of width substantially less than the diameters of said inner conductors secured along one edge to the inner conductor of said first section, said strip extending longitudinally along the inner conductor opposite the coupling slot, a second thin conductive strip of width substantially less than the diameters of said inner conductors secured along one edge to the inner conductor of said second section, said second strip extending longitudinally along the inner conductor of said second section opposite the coupling slot, said strips projecting radially toward each other, the ends of said strips being tapered, the width of said coupling slot being substantially greater than the thickness of each or" said strips.

3. An ultra-high-frequency directional coupler of the coaxial transmission line type comprising a first section of coaxial transmission line having an inner and outer conductor, a second section of coaxial transmission line having an inner and outer conductor, said second section being oriented parallel to the first section and having a common outer conductor wall section therewith, said common wall section having an axially directed coupling slot located. therein, the width of said coupling slot being greater than the diameters of said inner conductors, and a thin conductive bar secured along on edge to the inner conductor of said first section of coaxial line opposite the coupling slot and projecting radially toward the slot substantially in the plane defined by the parallel inner conductors of said coaxial sections, the bar having a width less than the diameters of said inner conductors.

4. An ultra-high-irequency directional coupler comprisinga first section of coaxial transmission line having an inner and outer conductor, a second section of coaxial transmission line having an inner and outer conductor, said second section being oriented parallel to said first section and having a common outer conductor wall section therewith, said common Wall section having an axially directed. coupling slot therein, the Width of said coupling slot being greater than the diameters of said inner conductors. a first thin 9 conductive strip of width less than the diameters of said inner conductors secured along one edge to the inner conductor of said first section, said strip extending longitudinally along the inner conductor opposite the coupling slot, and a second thin conductive strip of width less than the diameters of said inner conductors secured along one edge to the inner conductor of said second section, said second strip extending longitudinally along the inner conductor of said second section opposite the coupling slot, said strips pro- References Cited in the file of this patent UNITED STATES PATENTS Number Number Name Date Hansen July 30, 1946 Sheppard Jan. 13, 1948 FOREIGN PATENTS Country Date Great Britain Aug. 22, 1949 

